Taurine can be referred to as 2-aminoethanesulfonic acid and is one of the amino sulfonic acids found in the tissues of many animals. Taurine is an extremely useful compound with beneficial pharmacological effects, such as detoxification, fatigue-relief, and nourishing and tonifying effects. As a result, taurine finds wide applications as an essential ingredient for human and animal nutrition.
Taurine is currently produced in an amount of over 50,000 tons per year from either ethylene oxide or monoethanolamine. At the present time, most taurine is produced from ethylene oxide, following a three-step process: (1) the addition reaction of ethylene oxide with sodium bisulfate to yield sodium isethionate; (2) the ammonolysis of sodium isethionate to yield sodium taurinate; (3) the neutralization with an acid, i.e., hydrochloric acid and preferably, sulfuric acid, to generate taurine and inorganic salts.
Although the ethylene oxide process is well established and widely practiced in commercial production, the overall yield is not very high, less than 80%. Moreover, the process generates a large waste stream that is increasingly difficult to dispose of.
The first stage of the ethylene oxide process, the addition reaction of ethylene oxide with sodium bisulfite, is known to yield sodium isethionate in high yield, practically quantitative, as disclosed in U.S. Pat. No. 2,820,818 under described conditions.
Therefore, the problems encountered in the production of taurine from the ethylene oxide process arise from the ammonolysis of sodium isethionate and from the separation of taurine from sodium sulfate.
U.S. Pat. No. 1,932,907 discloses that sodium taurinate is obtained in a yield of 80%, when sodium isethionate undergoes ammonolysis reaction in a molar ratio of 1:6.8 for 2 hrs at 240 to 250° C. U.S. Pat. No. 1,999,614 describes the use of catalysts, i.e., sodium sulfate, sodium sulfite, and sodium carbonate, in the ammonolysis reaction. A mixture of sodium taurinate and sodium ditaurinate is obtained in a yield as high as 97%. However, the percentage for sodium taurinate and sodium ditaurinate in the mixture is not specified.
DD219023 describes detailed results on the product distribution of the ammonolysis reaction of sodium isethionate. When sodium isethionate undergoes the ammonolysis reaction with 25% aqueous ammonia in a molar ratio of 1:9 at about 280° C. for 45 minutes in the presence of sodium sulfate and sodium hydroxide as catalyst, the reaction products comprise 71% of sodium taurinate and 29% of sodium di- and tri-taurinate.
WO01/77071 is directed to a process for the preparation of ditaurine by heating an aqueous solution of sodium taurinate at a temperature of 210° C. in the presence of a reaction medium. A mixture of sodium taurinate and sodium ditaurinate is obtained.
It is therefore concluded from the foregoing references that the ammonolysis of sodium isethionate invariably yields a mixture of sodium taurinate, sodium ditaurinate, and sodium tritaurinate. The percentage yield of sodium taurinate has not been more than 80%.
In order to obtain taurine from sodium taurinate, U.S. Pat. No. 2,693,488 discloses a method of using ion exchange resins involving a strongly acid ion exchange resin in hydrogen form, and then an anion exchange resin in basic form. This process is complicated and requires the use of a large quantity of acid and base to regenerate the ion exchange resins in each production cycle.
On the other hand, CN101508657, CN101508658, CN101508659, and CN101486669 describe a method of using sulfuric acid to neutralize sodium taurinate to obtain a solution of taurine and sodium sulfate. Crude taurine is easily obtained by filtration from a crystalline suspension of taurine after cooling. However, the waste mother liquor still contains taurine, sodium sulfate, and other unspecified organic impurities, which are identified as a mixture of sodium ditaurinate and sodium tritaurinate.
In the co-pending application Ser. No. 14/120,046, a novel process is disclosed for converting alkali ditaurinate or alkali tritaurinate, or their mixture, to alkali taurinate.
U.S. Pat. No. 8,609,890 discloses a process of using isethionic acid or sulfur dioxide to neutralize alkali taurinate to producing taurine and to regenerate alkali isethionate. U.S. Pat. No. 9,108,907 further discloses a process of using isethionic acid prepared from ethanol to neutralize alkali taurinate to regenerate alkali isethionate. The aim is to reduce or eliminate the use of sulfuric acid as an acid agent in the production of taurine. However, the production yield remains low, due to the accumulation of impurities, which have now been identified as alkali ditaurinate and alkali tritaurinate.
It is, therefore, an object of the present invention to disclose a process for the production of taurine from alkali isethionate in a high overall yield (i.e., greater than 90% to nearly quantitative). According to the process of the present invention, a solution of alkali isethionate or regenerated alkali isethionate, alkali ditaurinate, and alkali tritaurinate is mixed with an excess ammonia and is subjected continuously to the ammonolysis reaction to form a mixture of alkali taurinate, alkali ditaurinate, and alkali tritaurinate, in the presence of one or more catalysts. After removal of excess ammonia, isethionic acid is used to neutralize the alkaline solution to yield taurine and to regenerate alkali isethionate.
The advantage of using isethionic acid as an acid becomes apparent in that no isolation of alkali salt is necessary after separation of crystalline taurine from the mother liquor containing alkali isethionate, alkali ditaurinate, and alkali tritaurinate.